In motor vehicles having a modern automatic transmission or an automated mechanical transmission, the utilization of the intelligence in the electronic system finds its expression in the use of an electronic transmission control with which the known criteria relevant for customers, such as driving comfort and driveability, and the criteria relevant to safety are to a great extent satisfied. For gear or ratio selection, depending on the situation, the electronic transmission control steadily communicates with other control units and computers of different aggregates of the motor vehicle, especially with an electronic control, usually via a CAN (Controller Area Network) database.
In case of a defect in the transmission or in the transmission control, as well as malfunction of the CAN database, emergency programs or alternative programs (alternative functions) are laid out so that in an emergency operation, the motor vehicle can be operated in the widest possible speed range, but sufficient mechanical and technical transmission protection are ensured even under extreme operation conditions.
From the practice different drafts for reducing the mechanical and thermal load of one such transmission in an emergency operation are already known. DE 198 44 618 A1, for example, has disclosed a method for reducing the thermal load of an automatic transmission of a motor vehicle in an emergency operation in which during the emergency operation of the transmission, a program module of an electronic engine control, independently of the transmission presets an admissible maximum engine torque for the emergency operation. As an admissible maximum engine torque for the emergency operation, a constant value can be preset here. However, in a complex function, the admissible engine torque can also be preset, depending on an engine rotational speed, in such a manner that the admissible engine torque in a rotational speed range between zero or engine idling speed up to the stall rotational speed of the torque converter, is reduced to a defined stall speed value and in a range of the engine rotational speed between stall speed and a maximum engine rotational speed admissible for the emergency operation, a maximum engine torque corresponding to the normal operation is admitted and in a range above the maximum engine rotational speed admissible for the emergency operation is completely ruled out.
This definition of the driving torque of the vehicle which also leads to a definition of the vehicle speed constitutes an easy, but effective step against thermal overstress and wear due to heat of components of the automatic transmission and against premature thermal aging of the oil, both in relation to an electrohydraulically controlled multi-step automatic transmission of conventional type and in relation to a CVT transmission. It is clear to the expert that this can also be similarly transferred to an automated mechanical transmission, such as a double-clutch transmission, insofar as this transmission has at its disposal an emergency gear draft. In particular, this protective function can also be used in relation to a converter automatic transmission as mechanical protection in the emergency operation, in case the possible transmission input torque, as result of the torque increase specific to the converter, can exceed an admissible input torque specific to the transmission. An inadmissible overstepping of the admissible transmission input torque in normal operation of the transmission being prevented by a reduction of the prime mover torque initiated by the electronic transmission control, when needed.
Corresponding to the kind of the existing emergency operation of the transmission, that is, depending on whether there is a “genuine” emergency program with completely deficient electronic control with the consequence of an extensively restricted driveability of the motor vehicle or whether there has been activated only an alternative driving program specific to the deficiency or an alternative function specific to the deficiency with the consequence of a more or less limited driveability of the motor vehicle. Corresponding to the type of torque reduction of the prime mover, a reduction of the performance of the motor vehicle results in the emergency operation of the transmission, especially of the starting performance, clearly traceable by the driver.
When driving, the motor vehicle during emergency operation of the transmission, there arises the following problem: the driver naturally associates his accelerator pedal position with a performance appearing on the strength of the driver learning the relationship between accelerator pedal angle and performance in the normal driving operation. But, in the emergency operation of the transmission, the learned relationship no longer exists as a result of the torque limitation on the side of the prime mover. In a natural manner, the driver will automatically try, when starting the emergency operation, to immediately compensate the allegedly deficient engine performance related to his accelerator pedal position by a strengthened accelerator pedal actuation or a larger accelerator pedal angle. The driver also reacts similarly in natural manner to a reduced performance after starting in emergency operation. In emergency operation, he will always preset a somewhat larger accelerator pedal angle than in the normal operation of the transmission when the actually available performance does not correspond to his expectations.
Especially when the emergency operation is combined only with limitations of the driveability of the motor vehicle not significantly traceable for the driver (such as quite possible in an alternative driving program), this emergency operation can also be activated over a longer period of time or a longer distance with the consequence that the driver becomes accustomed to the reduced performance of the motor vehicle compared to the normal operation and also revises the originally learned interrelation between accelerator pedal actuation or accelerator pedal position and performance.
If there now occurs a sudden change from emergency operation back to normal operation of the transmission, the driver is startled, especially by the starting performance of the motor vehicle again available in full force. His accelerator pedal actuation or accelerator pedal position adapted to the previous emergency operation still activated produces an occasionally clearly higher vehicle acceleration than expected. A change from the emergency operation back to normal driving operation of the transmission can occur, for example, after a new start of the prime mover or after a supply voltage reset of the electronic transmission control, but also, for example, automatically during the driving operation with permanent voltage supply of the electronic transmission control in activated or not activated gear of the transmission. On account of the highest possible availability sought of the motor vehicle, a change from emergency operation back to normal operation of the transmission is suitable in itself and also desired and can result, for example, always when a previously diagnosed deficiency of the transmission, of the transmission control, or of the data communication between transmission control and engine control no longer exists. However, as shown, this can also lead to driving situations critical to safety.
The problem on which this invention is based is to provide an automatic or automated transmission of motor vehicle and a method for control of the transmission in which, in an emergency operation of the transmission, the performance of the motor vehicle is limited compared to a normal operation and in which, when changing from emergency operation back to normal operation of the transmission, driving situations critical to safety are prevented.